Tunnel to be drilled under Magnolia bluff this summer

King County WTD will drill a pipeline underneath the Magnolia bluff this summer and fall to direct water overflow to the new tank in Smith Cove Park. Photo courtesy of King County Wastewater Treatment Division

This summer, a tunnel will be drilled below the Magnolia bluff to connect the existing sewer pipes and the new storage tank for the South Magnolia Combined Sewer Overflow (CSO) project from King County Wastewater Treatment Division (WTD).

During major rainstorms, the existing diversion structure cannot handle the increased water, and a mixture of rainwater and raw sewage overflows directly into Puget Sound. This happened 36 times last year; now, WTD is under a federal consent decree to limit that to one time per year on a long-term average.

To get this water to the new storage facility, it will need to go via gravity pipe under the Magnolia bluff. The construction crew will bore using horizontal direct drilling (HDD). To do this, the workers will start with an 8-inch pipe and make the tunnel from beginning to end, keeping it full of drilling mud; then they will expand the tunnel to its full 42- to 48-inch diameter and pull in the fully assembled pipe. The pipe will go down a 1.7-percent grade. It will be about 130 feet underground at the deepest point along the 2,500-foot-long tunnel.

Most of this new equipment will be underground, with a crew pull-off area, some ventilation pipes and a monitoring panel above-ground at 32nd Avenue West.

The crew conducted a geo-technical survey to determine the best alignment for this boring. The material there is part-dense glacial rock, which similar to concrete, and part material that’s similar to silt.

The boring process will take about four to five months, according to Kim Staheli, trenchless engineer for the project.

This technology is very different than the Bertha boring technology currently stuck on its path to make the Highway 99 tunnel. HDD is very suited to the area, Staheli said, because they can pull the drill back, and it will never be stuck.

The construction for the pipe will likely begin this summer.

The plan is to have the pipe completed before the storage facility is done. Once that happens, the crew will connect the two and begin operation.

The facility will have automatic controls to accept and divert water. The new tank will be remotely monitored by West Point WTD staff, with crews on stand-by just in case, said WTD’s Karl Zimmer.

Local impacts

One of the biggest neighborhood impacts for this project will occur when 32nd Avenue West will close to foot and car traffic for approximately two days. Early estimates are that this will happen in late October.

There will be a two- to three-week notice ahead of that time. WTD representatives are currently meeting with affected residents to determine what their needs will be during that time, said WTD community-relations person Monica Van der Vieren. (To contact WTD about possible accommodations, contact Monica Van der Vieren at Monica.VanDerVieren@kingcounty.gov or the project hotline at (206) 205-0968.)

Right now, construction hours are set for 7 a.m. to 7 p.m. Monday through Saturday. There will be other impacts in the 32nd Avenue West area, including truck traffic, traffic shifts or detours and work to minimize dust and dirt.

There is always a concern about landslides in the area, Staheli said, which the team took into account. But because the tunnel is deep and small, there shouldn’t be any effect. Neighbors will hear a humming near the initial equipment site, but they won’t hear anything as the boring machine progresses underground. However, they may feel vibrations.

The construction crew also needs to consider a nearby eagle’s nest. From Jan. 1 to April 15, the workers will follow specific noise guidelines in a large area as to not affect at least one pair of eagles with an eaglet. King County staff will monitor the area, said environmental planner Hilary Schafer.